Single phase alternating current electric motors conventionally are provided with two windings on a stator core. These windings are inductively coupled to the rotor of the motor. Such motors are widely used for various purposes and range in size from very small fractional horsepower motors on up to multiple horsepower sizes. Single phase motors are particularly popular, since most home and business alternating current supplies are in the form of single phase power.
Single phase electric motors include a stator core, which is wound with a start winding and a run winding connected to the source of operating power. These windings surround and are inductively coupled to a rotor which rotates a shaft to produce the motor output. Rotors are made in a number of different configurations, such as squirrel cage rotors, high resistance rotors, low resistance rotors, wound rotors, or multiple winding high and low resistance rotors. All of these configurations, along with various stator winding arrangements, are well known in the electric motor industry.
Typically, the start winding is made of relatively small diameter wire; and the run winding is made of equal or relatively large diameter wire, compared to the diameter of the start winding. These windings are physically and electrically angularly displaced from one another on the stator.
In conventional capacitor-start and capacitor-start/capacitor-run motors, a starting capacitor is connected in series with the starting winding and a switch. At motor start-up, the switch is closed and the capacitor, in conjunction with the relatively small diameter starting winding, produces a leading current in the starting winding which is approximately equal to, and approximately 90.degree. displaced in phase from, the lagging current in the main or run winding of the motor. Such arrangements produce high values of starting torque.
Usually, a conventional capacitor-start motor has a centrifugal or thermal switch connected in series with the capacitor and start winding across the input terminals. The run winding is connected in parallel with this series-connected starting circuit. The starting condition is such that the instantaneous locked rotor current is high; and the motor starting current demand factor also is high. As a consequence, such motors undergo relatively high operating temperatures. Because the starting winding of such motors generally is a relatively small diameter wire, overheating can and frequently does occur. Such overheating results in a relatively limited life of the starting winding due to burnout, particularly under overload conditions of operation of the motor.
Applicant has developed capacitor-start/capacitor-run motors which do not use small diameter starting windings, but instead utilize two series-connected windings (of substantially the same diameter heavy wire) electrically phase displaced 90.degree. from one another on the stator core.
Applicant's U.S. Pat. Nos. 4,734,601 and 4,772,814 disclose motors in which one of the windings has a capacitor connected in parallel with it to form a parallel resonant circuit at the operating frequency of the motor. These motors are high efficiency motors which overcome most of the disadvantages of the prior art capacitor-start/capacitor-run motors. The motors disclosed in both of these patents, however, have relatively low starting torque. Consequently, such motors primarily are suitable for use in situations which do not require very high starting torques, such as pumps, blowers, machine tools and many commercial and domestic appliances.
For situations where higher starting torques are required, a variation of the parallel resonant configuration has been developed. This is disclosed in applicant's U.S. Pat. No. 4,675,565. The motor disclosed in this patent also use a parallel resonant circuit at the operating frequency of the motor. In addition, however, a second capacitor is connected in series with a switch in parallel with the first capacitor. This switch is closed during start-up of the motor, and is opened during normal load conditions of operation of the motor. This permits a substantial increase in the starting torque of the motor. During normal operating or running conditions of the motor when the switch is opened, the parallel resonant circuit functions in the same manner as disclosed in the motors of U.S. Pat. Nos. 4,734,601 and 4,772,814.
Applicant also has developed a motor with improved starting torque which utilizes a series resonant circuit formed by the run winding and a capacitor having a high capacitance. The start winding is connected in parallel with the series connected run winding and capacitor. This system is disclosed in applicant's U.S. Pat. No. 4,794,288. During full load and no load running conditions of operation of the motor, the major portion of the current passes through the run winding and capacitor, with lower current (approximately 25% to 50%) flowing through the start winding.
All of the motors disclosed in the above-identified patents are designed for single speed operation. Multiple-speed single phase induction motors typically include internally connected motor switches in the form of centrifugal switches, relays, or the like. Such internally located switches frequently produce arching and additionally require space for accommodating such switches. The location of internally connected motor switches also subjects the switches and the mechanism for operating them to increased temperatures from the motor windings, particularly when the motor is operating under heavy load and start conditions.
Two prior art multiple-speed single phase induction motors, of the type mentioned above, are disclosed in the patents to Schaefer U.S. Pat No. 1,961,793 and Michelsen U.S. Pat. No. 2,068,559. Both of these patents disclose the use of centrifugal switches for disconnecting a relatively small-sized start winding from the operating circuit for the motor after the motor attains a pre-established running speed. In the motor disclosed in the Schaefer patent, the motor always is first started in its high-speed mode. After start-up has been effected and the centrifugal switch has removed the starting winding, an external selector switch which, can be operated to select either medium or low speed windings for the running operation of the motor. It is always necessary to start the motor in its high-speed mode.
In the motor disclosed in the Michelsen patent, the motor always must be started in its low-speed mode. After a minimum running speed has been attained, the centrifugal switch removes the starting winding from the circuit. The speed selection switch then can be moved to either an intermediate or high-speed winding for running the motor. In both Schaefer and Michelsen, the centrifugal switch is operated to remove the starting winding from the operating circuit of the motor once the operating speed of the motor has been attained.
Accordingly, it is desirable to provide a multiple speed induction motor of the general types described in the above-mentioned prior art patents which overcomes the disadvantages of such patents and which utilizes switching controls located externally of the stator rotor and the associated operating windings of the motor.